feat: suggest columns in SELECT before FROM clause is written

[datlechin]

Summary

• Column autocomplete now works before a FROM clause is written, suggesting columns from all cached schema tables as fallback (matching DataGrip/SSMS behavior)
• Eager column cache warming via driver.fetchAllColumns() runs in background after schema loads, so columns are instantly available
• Schema-qualified table names (e.g. public.users) now correctly resolve in autocomplete regex

Changes

SQLSchemaProvider.swift

• Eager column loading: after loadSchema() fetches tables, a background Task pre-fetches all columns via fetchAllColumns() and populates the LRU cache (capped at maxCachedTables)
• New allColumnsFromCachedTables(): zero-network fallback that reads only from cache, deduplicates column names (unique → plain name, ambiguous → table.column), and returns items with sortPriority = 150

SQLCompletionProvider.swift

• New columnItems(for:) helper: single decision point that dispatches to allColumnsInScope (when table references exist) or allColumnsFromCachedTables (fallback). Replaces 13 call sites.

SQLContextAnalyzer.swift

• tableRefRegexes updated from [\\w]+ to [\\w.]+ to handle schema-qualified names like public.users
• New stripSchemaPrefix() strips schema prefix using NSString O(1) operations
• sqlKeywords promoted to static let tableRefKeywords (avoids per-call allocation)

SQLCompletionAdapter.swift

• .select added to browseable context list with comma guard — auto-shows completions after SELECT but not after each comma in the column list

Test plan

• 559 autocomplete tests passing, zero regressions
• Connect to a database, type SELECT — verify column suggestions appear
• Type SELECT na — verify columns matching "na" from all tables appear
• Type SELECT * FROM users WHERE — verify columns from users appear (existing behavior preserved)
• Type SELECT col1, — verify popup does NOT auto-trigger after comma
• Type SELECT col1, n — verify columns appear when typing after comma
• Test with schema-qualified table: SELECT * FROM public.users — verify columns resolve correctly
• Test with large schema (50+ tables) — verify no performance issues

Closes #840